The Trackball is supported up to some grade by the Mouse control panel, but is not recognised by the system as a standard mouse. It might work partially on any original ADB equipped Mac without specific software. To take full advantage of the device the Kensington MouseWorks software should be installed, but this software is picky about the system software version you use. Check any version available for your system, beginning with the most recent one. On a beige G3 with Mac OS 9 or 10.2 it should work fine with appropriate software, as this machine is factory equipped with an ADB. On more recent Macs up to Mac OS 10.3.9 the Griffin iMate might be a viable solution as it comes with some software to support devices that lack support from Mac OS X inbuilt drivers. On a Mac Mini running Mac OS 10.4 the system seems not to recognise any ADB pointing device. (Setup with Mac Mini PPC , iMate ADB-USB-adaptor, NO GO with or without Griffin iMate software or USB Overdrive.)

Edit: NO GO with Mac Mini / 10.4.11 / iMate / standard ADB Mouse II, but ADB Keyboard still works. I did not notice the lack of any ADB mouse support as I always use an USB Glidepoint with this machine.

In case you already have installed Kensington MouseWorks check the settings: you can tune the tracking speed within a very wide range and for each application individually. And have a close look at the cable. A broken wire could cause a lot of trouble and randomised response.